1. Field of the Invention
The present invention relates to a disc-shaped information recording medium comprising an optical penetrating substrate having a colored underlayer and an ink receipt layer in this order on the principal surface of one side of the substrate, and a method for manufacturing the same.
2. Description of the Related Art
CD (Compact Disc) has spread in the field of audios. Further, with the prevalence of personal computers, CD-ROM for read-only, write once type CD-R capable of recording information only one time on one area, and CD-RW capable of rewriting information records any number of times are used as optical media for the storage of data.
CD-ROM comprises, for example, a transparent substrate having a diameter of 120 mm and a thickness of 1.2 mm, on which pit rows are formed by a track pitch of 1.6 μm, and has storage capacity of about 650 M-byte, and information is reproduced by irradiation with laser beams having wavelengths of from 770 to 790 nm at a constant linear velocity of from 1.2 to 1.4 m/s.
With the development and practical use of short wavelength lasers in recent years, it is contrived to spread DVD (Digital Versatile Disc) capable of higher density recording and reproduction, and disc-shaped information recording media coping with HD, DVD specification and BD (Blue ray Disc) specification capable of further higher density recording and reproduction by the development and practical use of what are called violet lasers that are further shorter wavelength lasers.
Similarly to the case of CD, DVD-ROM, DVD-R, DVD+R and DVD-RW are exemplified as DVD. DVD-ROM has recording density of about six to eight times that of CD, and has construction of sticking two substrates having a thickness of 0.6 mm or so, wherein a pit is formed by track pitch of 0.74 μm, and information is reproduced by irradiation with laser beams having wavelengths of from 635 to 650 nm at a constant linear velocity of 3.5 m/s or so.
CD and CD-ROM are manufactured by using a resin such as polycarbonate and the like, and has a structure comprising a doughnut-shaped disc as the optical penetrating substrate having thereon a reflective layer formed by vapor deposition with gold, silver or aluminum, and a protective layer formed of an ultraviolet curing resin and the like on the reflective layer. For recording, rugged pit rows are formed on the surface of the optical penetrating substrate according to spiral-shaped array. These pit rows are formed in advance by a mold such as a stamper and the like at the time of forming the optical penetrating substrate, and the reflective layer is provided thereon.
The fundamental structures of DVD and DVD-ROM are the same as those of CD and CD-ROM, but the thickness of the optical penetrating substrate of DVD and DVD-ROM is half of the thickness of CD, and the thickness equal to that of CD as the thickness of the disc as a whole is secured by sticking two optical penetrating substrates half as thick as a CD to each other. For example, in the most ordinary DVD and DVD-ROM of a single-sided layer, rugged pit rows are formed on an optical penetrating substrate and a reflective layer is provided thereon, and the substrate is stuck to a substrate having no such pit rows and a reflective layer.
In recent years, in such disc-shaped information recording media, an information recording medium capable of drawing with water color ink by means of a water color ink pen and an ink-jet printer has been developed by providing a colored underlayer and an ink receipt layer receiving water color ink in this order on the side of an optical penetrating substrate opposite to the side from which record light and reproduction light are incoming. This kind of information recording media conventionally has an ink receipt layer capable of fixing water color ink on the surface of a protective layer or on the surface of a stuck substrate. Ink receipt layers are divided broadly into layers comprising an ultraviolet curing property resin capable of absorbing ink and expanding, and layers mainly comprising inorganic particles such as a pigment and a binder, and having many gaps and absorbing ink into the gaps. Many of information recording media equipped with an ink receipt layer capable of high quality drawing that is called photograph image quality have the ink receipt layer having many gaps on a colored underlayer as described above, and capable of drawing good in water resistance by absorbing the ink coated by means of a water color ink pen and an ink-jet printer into the gaps.
An example of manufacturing methods of information recording media comprising an optical penetrating substrate having a colored underlayer and an ink receipt layer in this order on the principal surface of one side of the substrate is disclosed in JP-A-2006-102993 (the term “JP-A” as used herein refers to an “unexamined published Japanese patent application”). In order to provide an information recording medium having an ink receipt layer capable of printing in high image quality, little in bleeding, and not accompanied by the occurrence of warp, in the manufacturing method of the disc-shaped information recording medium of forming the ink receipt layer by coating a coating liquid for the ink receipt layer containing fine particles, a binder and a crosslinking agent on the underlayer, the surface tension of the coating liquid for the ink receipt layer is prescribed to be 5×10−2 N/m or less. Specifically, the coating liquid for the ink receipt layer containing vapor-phase method silica as the fine particles having an average primary particle diameter of 7 nm, polyvinyl alcohol as the binder, boric acid as the crosslinking agent, and polyoxyethylene lauryl ether as the surface active agent is coated on the underlayer of the information recording medium, and dried to form the ink receipt layer. Further, a disc-shaped information recording medium having an ink receipt layer using vapor-phase method alumina as fine particles is disclosed in JP-A-2006-260748.
As described above, in disc-shaped information recording media having a gap-type ink receipt layer on a colored underlayer, ultraviolet curing property resins or thermosetting resins are generally used as the materials of the ink receipt layer, but when a thicker ink receipt layer is formed on the colored underlayer, greater energy must be supplied at the time of ultraviolet curing or thermosetting time, and there are cases where warp and flutter of the disc-shaped information recording media are caused. If warp occurs in disc-shaped information recording media like this, there is a possibility that the recording characteristics of the disc-shaped information recording media are influenced, although it depends upon the degree of the warp.
In the background art disclosed in JP-A-2006-102993, a wetting property of the pore surfaces of the gaps is improved by the addition of a surface active agent to the coating liquid for ink receipt layer to restrain the occurrence of warp of the disc-shaped information recording media, but it has been difficult to stably obtain restraining effect of warp dependent on preservation conditions.